To achieve the complexity of cell types found in the mature brain, different classes of neurons are generated from germinal zones in the embryos. This project aims to elucidate how such production sequences arise, with emphasis on the cell fate specification events critical to the generation of a family of brainstem precerebellar neurons. The recombinase fate mapping system generated in the Dymecki lab provided evidence that supports the model that precerebellar neurons are specified, in part, in the rhombic lip. This proposal focuses on the role of the transcription factor Pax6 in the specification of the precerebellar system. Loss of Pax6 results in the absence of three of the six precerebellar nuclei. I hypothesize that the identities of the precerebellar neurons are specified, in part, in the rhombic lip and that critical "specification genes" expressed in the rhombic lip are regulated by Pax6. This will be examined in Aim 1 by identifying on a genome-wide level Pax6-targets expressed in the rhombic lip. These experiments will be complemented with those proposed in Aim 2 which use the established recombinase-based fate-mapping tools of the Dymecki lab to map rhombic lip histogenesis in Pax6-null mice.